1. Technical Field of the Invention
The present invention relates generally to communication systems and more particularly to communication systems that utilize assisted Global Positioning System (GSP) data and/or Satellite Based Augmentation System (SBAS) data to determine positioning data.
2. Description of Related Art
Global Positioning System (GPS) is well known and widely used. For example, GPS is used for vehicle navigation systems, personal locaters, exercise odometers/speedometers, aviation, etc. While there are a wide variety of applications of GPS, there are currently two primary issues. One issue relates to the relative inaccuracy and unreliability of GPS receivers for aviation and the second issue is weak signal reception due to obstructions (e.g., buildings, garage structures, geography, etc.)
To address the first issue, the United States (e.g., Wide Area Augmentation System—WAAS), Europe (e.g., European Geostationary Navigation Overlay Service—EGNOS), and Japan (e.g., Multi-transport Satellite Augmentation System—MSAS) are each developing a compatible Satellite Based Augmentation System (SBAS) mainly to improve aviation safety. In general, an SBAS system is a satellite based differential GPS system (DGPS) with a reference network of GPS ground stations collecting and transferring error measurements to one or more ground central control units. The central control unit(s) calculates error correction terms and transfers them to geostationary satellites. The geostationary satellites transmit correction terms in “GPS-like” signals back to SBAS-enabled GPS receivers, which then use the data to correct for the errors. This is similar to Differential GPS (DGPS) except that in DGPS, each reference station calculates the error corrections individually and broadcasts it in its neighborhood, unlike SBAS where the correction terms are obtained in a central unit using multiple reference stations and are communicated to GPS receivers via geostationary satellites.
In one implementation of an SBAS system, a number of GPS receiver stations, which may also be referred to as RIMS (Ranging and Integrity Monitoring Stations), are positioned throughout a geographic area. The position of the GPS receiver stations must be precisely known (e.g., accuracy to within a few centimeters), which enables the RIMS to calculate the difference between the known position of the station and the position as calculated by the GPS receiver. Further, since RIMS receivers use both GPS frequencies (L1 and L2) the signal delay through the ionosphere can be calculated for every single satellite. Still further, if the signals from more than four satellites are received, more information than needed for a position determination is available, this information may be used to check for possible problems with the satellites or deviations in their orbits or time, which includes long term errors of the satellite orbits, short term and Long term errors of the satellite clocks, ionosphere (IONO) correction grids, and integrity information. From the integrity information, it is possible to inform users within six seconds of problems that occur with the GPS system. The IONO correction grid is used to correct for ionosphere signal delay, which is typically the largest source of error in GPS position determinations.
To address the second issue (i.e., weak signal reception due to obstructions), an assisted GPS system was developed. The assisted GPS system includes a network of reference GPS stations that provide assisting data to GPS-enabled mobile devices through a cellular network. Note that the reference GPS stations may or may not reside at the base stations. In one instance, the assisting data includes satellite almanac, satellite ephemeris, and satellite clock error information, which are derived from 50 Hz navigation data. In another instance, the assisting data may include the bits of the 50 Hz navigation data. By providing this information to a GPS receiver via a cellular system as opposed to the GPS receiver attempting to extract the information from a weak GPS signal, the receiver's acquisition time (or the Time to First Fix (TTFF)) is improved and the GPS receiver is relieved from the requirement to decode the data, which may increase the coherent integration interval and hence achieve considerably better sensitivity.
As is known, assisted GPS systems can be implemented in two different modes: mobile-assisted and mobile-based. In a mobile-assisted system, a mobile device that includes the GPS receiver processes received GPS signals to obtain only correlation outputs and/or raw measurements, which are then transmitted back to the base station where the complete navigation solution is obtained. In a mobile-based system, the complete navigation solution is obtained within the mobile and communicated back to the base station.
As is further known, the assisting data exchange may be done on either the control plane or the user plane of the cellular system. For the control plane exchange, an A-GPS server is integrated into the cellular network infrastructure and the assisting data is provided over the signaling channels. In this case, the A-GPS server connects to the Serving Mobile Location Center (SMLC) and the assisting data travels through the Mobile Switching Center (MSC) to the base station and ultimately to the GPS-enabled mobile device. For the user plane exchange, the A-GPS server is essentially independent of the cellular network infrastructure and the assisting data is provided through user plane communication channels. In this case, the A-GPS server connects to the A-GPS client on the mobile device simply through say the IP connection. For example, the data may simply travel through Gateway GPRS Support Node (GGSN) to the Serving GPRS Support Node (SGSN) to the base station controller and the base station and ultimately to the GPS-enabled mobile device. Note that standards have been developed to standardize the assisting message contents and signaling for both user plane and control plane.
While SBAS addresses the first issue and the Assisted-GPS addresses the second issue, a need still exists for a system that addresses both issues.